1. Introduction
Temperature is probably the most measured environmental parameter in the world. Global warming has dramatically increased the need for accurate temperature measurement of the environment. Temperature measurement is also required in numerous industrial and domestic applications. One important example is the temperature control of a PC microprocessor [1]. Based on the temperature information produced by a thermistor, the operations of both the microprocessor and the cooling system operation can be optimally controlled. Temperature control is also typically required in household appliances, such as refrigerators, coffee makers and electric ovens [2]. In addition, overheating protection is applied in several devices, such as motors [3] and batteries [4]. In consumer electronics, the main application is body thermometers, typically measured from the tympanic membrane in the ear [5].
It is only natural to consider whether temperature sensing, which has such ubiquitous applications and is of general interest, could be implemented in mobile handheld devices such as mobile phones. There are thermistor-based temperature sensors for a variety of products, including wristwatches and a few mobile phones as well. However, their performance is highly limited for a simple physical reason. If we put a thermistor inside a mobile device, we are able to measure the temperature of a localised volume within the device case. However, this does not necessarily correlate with the real room temperature due to two main reasons. First, while the environment has little contact with the reference thermistor, thermal contact from the device itself to the reference thermistor is strong. In addition, mobile device can contain heat sources, such as power electronics, which easily increase the temperature within the device. Naturally, the heating effect is greater close to the heat sources, but heat conduction leads the entire device to be affected. Placing the sensor on the cover's exterior and isolating it from the rest of the device would improve sensor contact to the environment, but complete isolation is practically impossible to achieve. In addition, a sensor that is located on the cover's exterior will still be vulnerable to heating from the user's hand.
An alternative for accurate environmental temperature measurement in mobile device application is infrared (IR) temperature sensing. IR sensing offers a non-contact method to measure the temperature of targets due to the fact that all objects whose temperature is above absolute zero emit IR radiation [6]. The target temperature sensing is based on the measurement of the IR radiation emitted by the object. The main advantages over thermistor-based measurements are as follows: the thermal signal from the target can be amplified by optics and the thermal signal from the device itself can be attenuated by an advantageous optical design and implementation.